2 x quad-core system is slower that 2 x dual core on FreeBSD

Tom Evans tevans.uk at googlemail.com
Tue Nov 20 08:47:07 PST 2007

On Tue, 2007-11-20 at 19:27 +0300, Alexey Popov wrote:
> Hi.
> <snip>
> After that I rebuilt with SMP GENERIC kernel and put on that server 2 
> times more requests that UP could handle. For the first time it worked 
> good. Then I increased load to 2.5 times more than UP. Immediately 
> Apache child count increased to MaxClients (24), most of them in RUN 
> state, and %sys became greater than %user (see attach). I think after 
> some threshold of load FreeBSD is paying more CPU time to the management 
> of running processes than to run them.
> Also I tried to halt CPUs by machdep.hlt_cpus sysctl, but in that case 
> %sys in top was still much greater than %user.

MaxClients of 24 seems very low for a 8 cpu box, running prefork MPM. On
our quad CPU boxes, running custom apache modules, we use 
  MaxClients 70
  MinSpareServers 5
  MaxSpareServers 15
  StartServers 20

Perhaps you are seeing high system load because the system is having to
maintain a lot of queued connections. Certainly, our load remains
in-between comfortable margins, except when heavily stressed.

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